Testing caps with a DIY ESR meter

ESR

There’s a problem with collecting old tube amps and vintage electronics – eventually the capacitors in these machines will die. It’s not an issue of a capacitor plague that causes new electronics to die after a few years; with time, just about every capacitor will dry out, rendering antique electronics defective.┬áThe solution to getting old gear up and running is replacing the capacitors, but how do you know which ones are good and which are bad? With [Paulo]‘s DIY ESR meter, of course.

An ideal capacitor has a zero equivalent series resistance, and failure of a capacitor can be seen as an increase in its ESR. Commercial ESR meters are relatively cheap, but [Paulo] was able to build one out of a 555 chip, a small transformer, and a few other miscellaneous components.

The entire circuit is built on stripboard, and if you’re lucky enough to find the right parts in your random parts bin, you should be able to build this ESR meter with components just laying around.

Comments

  1. Bill Jackson says:

    I have replaced many old can type dried out electrolytics by opening the can at the bottom joint and replacing the innards with a modern electrolytic cap of the right value. Modern caps are a lot smaller than the old time ones, and they I put it together and solder the seam with aluminum solder. It looks a lot better than having new caps dangling underneath. It is often hard to find high voltage caps, so you may have to hunt. Standard Radio in Long Island NY has a lot, but they are costly.

  2. Kaj says:

    I used a TL084 circuit that did quite a nice job.

    1 section sets up a dual-polarity PSU, the next is a 100khz oscillator – this feeds a wheatstone bridge where the capacitor is tested, and then it is amplified and fed to a meter movement.

    I first found it here: http://kakopa.com/ESR_meter/index.html

    The advantage of that design over this one is that you don’t need to wind the 2:1 transformer, it automatically detects DC leakage (and is indicated by a warning light), and allows greater flexibility in gain adjustments.

    I constructed mine with standard spacing banana jacks, so with a BNC adapter I can use a variety of off-the shelf probes – the coax seems to keep the measurements more stable too.

  3. kay says:

    Very beautiful project, this should be in one of those rat shack mininotebooks. Paulo, maybe show it to them and they’ll print leaflets or put it on their web site?

  4. Lee Hart says:

    A great little project! However, it should be noted that it doesn’t measure ESR; it measures total capacitive reactance (ESR plus the capacitor’s AC impedance). A perfect capacitor will still read an “ESR” with this meter that is really the capacitive reactance Xc=1/(2 pi f C).

    As the author notes, ESR dominates for seriously bad capacitors. However, the results will be misleading for many good capacitors. For example, it can’t tell the difference between a good low-ESR electrolytic for switchmode power supply filtering, and a junky but good high-ESR capacitor only good for 60 Hz filtering and low currents.

    An improvement is to replace the rectifier diodes with a 4016 CMOS analog switch. Connect its control lines to the 555 oscillator so it works as a synchronous rectifier. The in-phase AC voltage across the capacitor under test is from its ESR. The 90 degree out of phase AC voltage is from the capacitive reactance. A synchronous rectifier will cancel out the AC from the capacitive reactance.

    • Kaj says:

      Raising this thread from the dead….

      Would it be possible to use an n-channel FET to “gate” the output to the diodes (triggered by the oscillator), so that they only get the signal to rectify when it’s in-phase?

  5. strider_mt2k says:

    When it comes to electrolytic caps it’s easy: When in doubt, swap it out.
    I know there are folks that don’t always have that luxury, and for them this will help to steer clear of those heart breakers. (Tom Petty no withstanding of course.) but I have never liked having to re-use old electrolytic caps.

    The idea of putting modern caps inside the shells of old-timey ones is brilliant for folks trying to keep the visual appeal of really classic gear.

    • Phil in Oz says:

      Yep, agree with Strider; replace and be sure. That’s what I do.
      Modern caps in old cans? Hmmm, who’s gonna be lookin’ under the hood?
      To Bill Jackson; you can find cheap hv caps online no problems; Search on Vintage radio web sites and you will soon get a link or 2 to sellers of caps and all sorts of neat old time radio stuff.

  6. stevebb says:

    say you’ve made your own capacitor, as I did last year after getting sick of unreliable water level sensors using conduction. see http://stevebb.wordpress.com/home/water-variable-polarised-capacitor/
    it’s relies on the capacitor “drying out”/water level to change the capacitance from 2nF to 116 nF.
    I understand about ESR, and leakage, but ideally what additional characteristics should be measured? Hints/simple circuits to do so so would be most welcome.

  7. Bill Gander says:

    Very cool Paulo! This would be useful for the newcomer or old timer. Thanks for sharing :)

  8. Oren Beck says:

    An “Idea Capacitor?” Is that what fails in those with no Idea?

  9. hawkeye18 says:

    What happens if the caps in this ESR meter go bad…? lol

  10. Dave M says:

    More ideas for DIY ESR measurement:
    http://kripton2035.free.fr/esr-repository.html

  11. I used to be a TV and VCR repair technician and soon realised testing the value of a capacitor was not enough. Many electrolytic caps develop a high ESR value yet their capacitance remains within limits.
    An ESR meter is essential for fault finding.

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